1. Field of the invention
The present invention relates to systems and methods for controlling the movement of mechanical devices. More particularly, the present invention relates to the servo control of electromagnetic devices. Still more particularly, the present invention relates to the servo control of solenoids using the measurement of position and the approximation of position of the solenoid's armature to regulate movement of that armature. The present invention may be used in a variety of areas where lifting and/or propulsion is desired with minimum energy consumption.
2. Description of the Prior Art
A solenoid is a linear motor, inherently capable of efficient conversion of electrical to mechanical energy. In rotary motors, experience teaches that large size favors efficiency, and for a given size motor, the highest efficiency is obtained when there are very close clearances between stator and rotor parts and when operation is at high RPMs. Electrically speaking, a high frequency of magnetic reversals translates into a high rate of transfer of electromagnetic power. At low frequencies, resistive power losses wipe out efficiencies, while at constant magnitudes of peak magnetic flux, higher frequency translates into higher power transfer without significant increase in I.sup.2 R resistive power loss. To avoid the eddy current losses associated with high frequency magnetic fields, rotary motors employ laminations in magnetic steels, or high-resistivity ferrite parts. Steels have a large advantage over ferrites at moderately low frequencies (particularly below 1 KHz) in their ability to handle flux densities up to about 2 Teslas, compared to ferrites at up to about 0.5 Teslas. The 4-to-1 advantage in flux density translates into a 16-to-1 advantage in energy density and magnetic force. Translating the rotary motor rules into the realm of solenoids, one can expect that efficient operation is fast operation. A fast solenoid must have a low shuttle mass, or alternatively, shuttle inertia may be cancelled by resonating its mass with a spring at the design operating frequency (as is done, e.g., in tuned magnetic vibrators for aquarium diaphragm pumps and barber clippers). As the counterpart of close clearances in rotary motors, solenoids operate efficiently at very short operating strokes, relying on high force and high frequency of operation to raise the power/weight ratio. Short strokes are effective only if, at the end of a power stroke, the entire magnetic circuit closes with minimal air gaps--a matter of efficient design. For a solenoid shuttle in non-resonant operation, a short stroke translates into a short stroke time, amounting to operation at high frequency and a high rate of change of magnetic flux, ".PHI.," as the magnetic gap closes. A high rate of change of flux, i.e., a large "d.PHI./dt," translates into a high induced magnetic voltage in relation to resistive voltage. Induced voltage represents conversion between electrical and mechanical energy, while resistive voltage represents energy loss, so a large "d.PHI./dt" translates into high efficiency.
There are and will always be solenoids designed for utilitarian binary control operations, e.g., unlocking the downstairs front door: contexts where power efficiency is of minor importance and stroke length is a matter of feasibility and convenience, rather than a matter of efficient motor design. Magnetic steel solenoid parts are typically solid rather than laminated, because eddy current losses in dynamic operation are not a design consideration. Moving from the context of infrequent operation of a door latch to the very frequent operation of a print wire driver in a dot matrix print head, repetitive impact and consequent work hardening of the magnetic steel in a solenoid becomes a serious consideration. Magnetic materials for solenoids should ideally exhibit a low coercive force, i.e. a low inherent resistance to change in magnetic flux. In magnetic steels, low coercive force correlates with a large crystalline structure, attained through high temperature annealing to allow growth of large crystals. Annealed steels are mechanically soft and ductile, and their low-coercive-force property is described as magnetically soft. Repetitive stress and shock break up large crystals in steel, yielding a finer grain structure that is mechanically work-hardened and magnetically harder. Permanent magnets are optimized for high coercive force, or high magnetic hardness: the ability to retain magnetization against external influences. In solenoids, the mechanical work hardening of the steel takes place in a strong magnetizing field, leaving permanent magnetism in the solenoid circuit. The result is to cause the solenoid to stick in its closed position after external current is removed. This is a failure mode for print wire solenoids. A standard approach to keep solenoid parts from sticking is to cushion the landing at full closure, leaving an unclosed magnetic gap, typically through the thickness of the cushion material. This residual gap generates resistance to residual flux after power removal, reducing the tendency of the shuttle to stick closed. Residual magnetic gaps compromise efficiency in two ways: because the most efficient part of the magnetic stroke is approaching full gap closure, where the ratio of force to electric power dissipation is high; and because currents for maintaining extended closure must be made substantially higher to overcome the magnetic resistance of gaps.
Prior art techniques for servo control of solenoid motion and, more generally, magnetic actuation, are summarized well in the introductory section of U.S. Pat. No. 5,467,244, issued to Jayawvant et al: "The relative position of the object is the separation or gap between the control electromagnet and the object being controlled and in prior art systems is monitored by a transducer forming part of the control signal generator for the feedback loop. Such transducers have included devices which are photocells (detecting the interruption of a light beam by movement of the object); magnetic (comprising a gap flux density measurement, e.g. Hall plate); inductive (e.g. employing two coils in a Maxwell bridge which is in balance when the inductance of the coils is equal); I/B detectors (in which the ratio of the electromagnet coil current and magnetic flux is determined to provide a measure of the gap between electromagnet and object; for small disturbances the division may be replaced by a subtraction); and capacitative (employing an oscillator circuit whose output frequency varies with suspension gap)." Dick (U.S. Pat. No. 3,671,814) teaches magnetic sensing with a Hall sensor. In the succeeding description of "Apparatus for the Electromagnetic Control of the Suspension of an Object" Jayawant et al derive, from a generalized nonlinear electromagnetic model, a linearized small perturbation model for use in magnetic suspension of an object in the vicinity of a fixed target position. Specifically, they make use of what they call "I/B detectors" (see above quote) wherein the ratio of current "I" divided by magnetic field strength "B" provides an approximately linear measure of the magnetic gap. In text to follow, the ratio "I/.PHI." will be used in preference to "I/B" since inductive voltage measurements lead to a determination of the total flux ".PHI." rather than a local flux density "B." Specifically, as noted by Jayawant et al, the time derivative "n.multidot.d.PHI./dt" equals the voltage electromagnetically induced in a winding of n turns linked by the magnetic flux ".PHI.." Thus, time integration of the voltage induced in a coil yields a measure of the variation in ".PHI.," and additional direct measurement or indirect inference of "I" leads to a determination of the ratio "I/.PHI." used to close the servo loop. Where electrical frequency is substantially higher than the frequency associated with solenoid mechanical motion, the ratio "I/.PHI." is also the ratio of the time derivatives "(dI/dt)/(d.PHI./dt)," so that a measurement of the high frequency change in current slope "dI/dt," divided by the corresponding measured change in induced voltage across n windings, "V=n.multidot.d.PHI./dt," again leads to a measure of position. One recognizes, in this latter ratio measurement, a measure of the inductance of a solenoid. It is well known that inductance can be measured by determining the natural frequency of an LC resonator having a known capacitance "C," a technique identified in the final part of the quotation from Jayawant et al, above. By either ratio technique, i.e. involving either a time integral of induced voltage or a time derivative of current, one determines position without the use of sensors apart from means to extract measures of current and induced voltage from the coil or coils employed as part of the actuation device. While these relationships are needed building blocks in the conception of the instant invention, they are not an adequate basis for a servo system generating large mechanical motions and correspondingly large changes in solenoid inductance. First, there are limitations to the linearized small-perturbation models taught by Jayawant et al for controlling large solenoid motions. Second, dynamic stability problems would remain even with a more complicated and costly servo implementation using non-linear circuit models, e.g., computing position as the ratio of current/flux and force as the square of flux, instead of Jayawant's tangential linear approximations of the ratio and square law relations. Where solenoid control is based on driving a winding with a voltage V in order to control a position X, the system to be controlled is fundamentally third-order, involving a nonlinear first order system to get from voltage to change in magnetic force (since voltage controls the first derivative of current in an inductive solenoid, and current change generates force change without significant delay), coupled to a second order system to make the two hops force to change in velocity and from velocity to change in position. It is understood that servo control over a third order system is prone to instability since phase shifts around the control loop, tending toward 270 degrees at high frequencies, readily exceed 180 degrees over the bandwidth for which control is desired. Phase-lead compensation as taught by Jayawant et al adds 90 degrees of phase margin, bringing at best marginal stability to an efficient electromechanical system. If electromagnetic efficiency is very low, so that resistance R dominates over inductive impedance .omega.L up to the servo control bandwidth of .omega., then the third order nature of the system is not manifest where gain exceeds unity, and phase-lead compensation provides an ample stability margin. An example of such a low-efficiency system is found in Applicant's "Bearingless Ultrasound-Sweep Rotor" system (U.S. Pat. No. 5,635,784), where a combination of extreme miniaturization and lack of a soft ferromagnetic core places the transition from resistive to inductive behavior well into the kilohertz range. For the efficient actuation systems taught in the instant invention, the transition from resistive to inductive impedance can fall below 100 Hz. "Tight" servo control implies a relatively high loop gain over the bandwidth of significant mechanical response, implying a loop gain-bandwidth product well in excess of the bandwidth of significant mechanical response. A combination of high efficiency and tight control spell problems for loop stability, for even with single-pole phase lead compensation, minor resonances, e.g., from mechanical flexure, can throw the servo system into oscillation.
While Jayawant et al describe closed-loop servo control techniques applicable where perturbations in position from a fixed target position are small, Wieloch (U.S. Pat. No. 5,406,440) describes an open-loop control technique for reducing impact and mechanical bounce in solenoids used in electrical contactors. Prior art actuation had consisted of instantaneously applying to the solenoid winding the full voltage needed to close the contacts under all operating conditions, taking into account manufacturing variations in the spring preload holding the contacts open. The fixed actuation voltage was usually well in excess of the minimum requirement, and the result was actuation with excessive force and resulting severe contact bounce. Wieloch teaches to ramp the solenoid current up slowly so that when the magnetic force is just sufficient to overcome spring preload force and initiate motion, there will be little additional increase in average actuation voltage before the solenoid stroke is complete. Efficient current ramping is accomplished via a switching regulator, which applies a steadily increasing voltage duty cycle to the solenoid winding while winding current recirculates through a diode during intervals between driving voltage pulses. At a sufficiently high switching frequency, the inductance of the solenoid effectively smoothes the current waveform into a ramp. Similar switching regulation is found in preferred embodiments of the instant invention, but with greater control in order to overcome limitations in Wieloch's soft landing design. When a solenoid begins to close, the resulting "back EMF" due to armature motion tends to reduce electric current, in relation to gap, to maintain a constant magnetic flux, with the result that increases in force with gap closure are only moderate. (The simplified model of Jayawant et al, equation 9, implies no change at all for force as a function of gap closure at constant magnetic flux. In the specification below, Eq. 42 corresponds to equation 9 except for the slope function "dx.sub.eff /dx," which Jayawant takes to be unity and which departs significantly from unity for moderate to large magnetic gaps, as indicated, e.g., in the approximate formulation of Eq. 20 of the following specification.) If a constant average voltage is applied to the winding (e.g., via constant duty cycle voltage switching at high frequency) and current begins to decrease with gap closure, then the current-limiting effect of resistance is reduced as current is reduced, so that the magnetic flux begins to rise. This can lead to an acceleration of a solenoid armature toward impact at full closure, depending on inductive time constants, mechanical inertia, and spring rate. Even under conditions where sufficiently soft landing is achieved, it is at the cost of a substantial excess energy consumption to generate a long ramp of pulse duty cycle and current, only the middle portion of which causes actuation. Adaptive adjustment of a pulse width or a pulse duty cycle during solenoid closure will be shown (below) to achieve soft landing under variable conditions with nearly the minimum net expenditure of electrical energy dictated by the given operating conditions.
Hurley et al (U.S. Pat. No. 5,546,268) teach an adaptive control device that regulates electric current to follow a predetermined function of the measured solenoid gap, in order to achieve a predetermined pull curve of the electromagnet. Though such a system responds to some of the limitations of Wieloch, it is not readily adaptable to an actuation system that must respond to changing conditions of starting position and the load force curve while achieving quiet, impact-free, efficient operation.
Both for controllability and energy efficiency, some solenoids have been designed with a region of operation in which stator and armature components have closely spaced parallel surfaces and the armature moves in-plane through a region of changing overlap, yielding a region of relatively constant actuation force at constant current. Eilertsen (U.S. Pat. No. 4,578,604) teaches such a geometry in a dual-coil device for linear mid-range actuation and a strong holding force at either end of the actuation stroke. Rotary actuation designs accomplish similar linearity properties using rotary overlap of parallel magnetic plates. The touchdown region where magnetic parts close in contact is commonly avoided in servo control contexts. Magnetic characteristics in this region have presumably been considered too nonlinear for practical control. In particular, the region of operation approaching full closure and contact of mating magnetic surfaces presents a very steeply changing inductance and correspondingly steep change in the sensitivity of force to change in coil current. For a solenoid operated below core saturation, the variation in magnetic force "F" with coil current "I" and magnetic gap "x" is described approximately by the proportionality "F .varies.(I/x).sup.2." When the gap in a solenoid reaches mechanical closure, the "x" denominator in this proportionality goes nearly to zero, implying a nearly singular relationship between the control variables and the resulting magnetic force. Interpreting published families of static force/stroke/voltage curves exhibiting approximately this proportionality equation, the engineer is likely to conclude that a position servo control loop becomes unmanageably nonlinear over wide actuation ranges or on approach to full magnetic closure of the solenoid. As evidence of the prevalence of this assumption, FIG. 2 of the recent Jayawant patent (U.S. Pat. No. 5,467,244) illustrates the proportionality "F .varies.(I/x).sup.2 " for magnetic force as a function of distance and indicates a small region, designated by the symbol ".DELTA.," over which the curve is comparatively linear and amenable to linear control techniques, which are subsequently disclosed. What has gone unrecognized is that a reformulation of the control problem leads to division of the system into two well-behaved, coupled subsystems: a fast first-order controller using voltage to control magnetic force, and a slower second-order position servo using the force-control servo. The major system nonlinearities are confined to the robust first-order controller subsystem. Thus, from a control standpoint, there remains no advantage to magnetic geometries that linearize the relationship of force to armature motion, whereas one can now capitalize on the advantages of mechanical simplicity and economy in solenoid geometries that involve the mating of flat surfaces. Such simple geometries are found in the patent literature going back many years, e.g., to Kussy (U.S. Pat. No. 3,324,356). Such geometries give a strong nonlinearity of force with gap at constant current, which needs to be countered by appropriate controller design if the mechanical economies of flat geometries are to be realized.
Holding currents or drive voltages for solenoids are commonly set well below the peak currents or voltages needed to get a solenoid moving toward closure. Both drive and holding signal levels must, in open loop systems, be set high enough to insure closure followed by holding under all conditions, including variability in manufacture from unit to unit, including variability of power supply source (e.g., utility line voltage), and including variability in the mechanical load. Closed loop solenoid control offers a way to reduce both drive and holding signals to minimum practical levels. Yet problems with stability and non-linearity inherent to magnetically soft ferromagnetic-core solenoids have impeded the development of servo solenoids, and therefore have prevented the potential efficiency advantages just described.
Solenoids have the potential for operating characteristics now associated with efficient motors: quiet impact-free operation, very frequent or continuous motion, and high efficiency at converting electrical energy to mechanical work. Reciprocating power from electricity is traditionally derived from a rotating motor and a cam or crank shaft, yet solenoids have been demonstrated, in the instant invention, to deliver reciprocating power at high efficiency, provided that the solenoid is designed to operate fast, in order to generate rapid changes of magnetic flux in its windings. In many reciprocating power applications, a solenoid with sophisticated control can offer greater simplicity and substantially tighter control than is achieved with a rotary motor and rotary-to-reciprocating motion conversion device. In the realm of control and sensing of external processes via a solenoid, the invention to be disclosed below can be configured to operate as a controller of position and simultaneous sensor of force, or as a controller of force and simultaneous sensor of position, or in an intermediate mode as a source of mechanical actuation with electrically controlled mechanical impedance characteristics, especially of restoration and damping. With rotary motors, such control has involved the use, e.g., of stepper motors used in conjunction with torque or force transducers, or of non-stepper motors used in conjunction with rotary position encoders and possibly torque or force transducers. The following specification will show a solenoid operated as the linear motor to drive a high-efficiency reciprocating pump, while two additional solenoids control the pump's inlet and outlet valves. All three solenoids operate silently and efficiently under servo control. This new system goes beyond objectives described and claimed in Applicant's U.S. Pat. No. 5,624,409, "Variable-Pulse Dynamic Fluid Flow Controller," a system using valve solenoid actuators that are mechanically similar to the ones described below and that achieve volumetric flow regulation from a pressurized fluid source over a very wide dynamic range of pulse volumes and rates. The system described below replaces the volume measurement device of Applicant's earlier invention with a solenoid that provides active pumping actuation in addition to fluid volume measurement, inferred from the position of the solenoid pump actuator, where that position is determined from measurement of the resonant frequency of the solenoid drive winding with a capacitor.